1. Field of the Invention
The present invention relates to a polymeric organic semiconductor material having liquid crystallinity, an organic semiconductor structure and an organic semiconductor device formed from the organic semiconductor material.
2. Description of the Related Art
As a typical example of an element of an organic semiconductor device, a thin-film transistor (also called organic TFT) using an organic semiconductor as an active layer (hereinafter, referred to as an organic semiconductor layer) can be mentioned.
In this thin-film transistor, the organic semiconductor layer is formed by vacuum deposition process from molecular crystals represented by pentacene. It is reported that in a method of forming an organic semiconductor layer by vacuum deposition process, an organic semiconductor layer having high charge carrier mobility, which is greater than 1 cm2/V·s, can be obtained by optimizing film-manufacturing conditions (for example, see Y.-Y. Lin, D. J. Gundlach, S. Nelson, and T. N. Jackson, “Stacked Pentacene Layer Organic Thin-Film Transistors with Improved Characteristics”, IEEE Electron Device Lett., 18, 606 (1997)). However, generally in the organic semiconductor layer formed by the above-mentioned vacuum deposition process, a large number of grain boundaries easily occur in polycrystal state of aggregated fine crystals, and further, defects easily occur so that such grain boundaries and defects inhibit transportation of charge carrier. Accordingly, when an organic semiconductor layer is to be formed by vacuum deposition process, it is actually very difficult to form an organic semiconductor layer serving as an element of an organic semiconductor device continuously with uniform performance over a sufficiently broad area.
On the other hand, a discotic liquid crystal is known as a material showing high charge carrier mobility (for example, see D. Adam, F. Closss, T. Frey, D. Funhoff, D. Haarer, H. Ringsdorf, P. Schunaher, and K. Siemensmyer, Phys. Rev. Lett., 70,457 (1993)). In this discotic liquid crystal, however, carrier transportation is performed based on 1-dimensional charge carrier transport mechanism along column-shaped molecular alignment. Thus, there is a problem that it is difficult to apply industrially because strict control of molecular alignment is required. Up to now, there is no report on a successful example of a thin-film transistor using the discotic liquid crystal as a material of an organic semiconductor device.
It has been reported that a rod-shaped liquid crystalline material such as a phenyl benzothiazole derivative also shows high charge carrier mobility in a liquid crystal phase (for example, see M. Funahashi and J. Hanna, Jpn. J. Appl. Phys., 35, L703-L705 (1996)). However, there is still no report on a successful example of a thin-film transistor using the rod-shaped liquid crystalline material in an organic semiconductor layer. The rod-shaped liquid crystalline material occurs in several liquid crystal phases, and as the structural regularity of the liquid crystalline material is increased, the mobility of charge tends to be increased. However, when this material turns into a crystal phase of higher structural regularity, the mobility of charge is reversely decreased or not observed, thus naturally failing to exhibit the performance of a thin-film transistor.
To utilize the liquid crystalline material in a liquid crystal phase showing high charge carrier mobility, encapsulation thereof into a glass cell is necessary. Thus, there are restrictions in respect of device manufacturing. Further, such rod-shaped liquid crystalline material shows liquid crystallinity at relatively high temperatures so that it cannot be utilized in the vicinity of room temperature (around −10 to 40° C.).
When a polymer material in a molecular dispersion system is used as an organic semiconductor material, an organic semiconductor layer, which having uniform charge carrier transfer property over a large area, can be formed by coating this organic semiconductor material. However, the charge carrier mobility of the resulting organic semiconductor layer is as low as 10−5 to 10−6 cm2/V·s, and is problematic because of its dependence on temperature and electric field.
To solve these problems, the present inventors have provided, in a previously filed application, an organic semiconductor structure having an organic semiconductor layer comprising at least partially an aligned liquid crystalline organic semiconductor material, wherein: the liquid crystalline organic semiconductor material has a core containing L-units of 6π-electron system aromatic ring, M-units of 10π-electron system aromatic ring, and N-units of 14π-electron system aromatic ring whereupon L, M and N each represent an integer of 0 to 4, and L+M+N=1 to 4; and having at least one kind of liquid crystal phase at its thermal decomposition temperature or lower.
However, the above-described organic semiconductor structure is formed from a liquid crystalline organic semiconductor material that is a non-polymer material, and with respect to the organic semiconductor material that is a polymer material, there is only the following example, and no effective organic semiconductor material has been found. And neither organic semiconductor structure nor organic semiconductor layer having effective charge carrier transfer property has been reported.
That is, a polymeric semiconductor material having high charge carrier mobility in the vicinity of room temperature has been reported conventionally, for example, by M. Redecker and D. D. C. Bradley (see M. Redecker and D. D. C. Bradley, Applied Physics Letters, vol. 74, 10, (1999)). It is reported in this literature that by using a polymer material having a long conjugated system as its main chain, heating this polymer material to a temperature at which it exhibits a nematic phase, and then quenching to torn a glassy polymer material wherein the nematic phase is fixed, a polymeric semiconductor material having high charge carrier mobility can be obtained. Particularly, it is reported therein that high mobility can be attained when the material is subjected to the above-described operation under a condition of being contacted with an alignment layer subjected to a rubbing treatment by rubbing a polyimide film.
Among the conventional materials described above, the carrier transport material or organic semiconductor material comprising a straight-chain-type polymer has been examined for industrial use by virtue of excellent coating property. To allow such carrier transport material or organic semiconductor material to exhibit high carrier transport ability, it is desirable that intermolecular hopping conduction subsidiarily occurs as well as mainly occurrence of intramolecular charge transportation in the main chain direction. Accordingly, intramolecular skeleton parts in the material are desirably conjugated with one another, which however gives rise to a problem of a limit to design of the material.
Moreover, to secure stability and reliability such as sufficient electrical property and longevity of the organic semiconductor element or the organic semiconductor structure, it is essential that the material forming the organic semiconductor element and the organic semiconductor structure are refined to desired purity. However, in the charge transport material and organic semiconductor material comprising a polymer, deriving from the polymer material structure, there are some limitations (for example, solubility, melting point, boiling point etc.) in means of purification. Therefore, there is a problem that industrially satisfying purity cannot always be secured in many cases.
Further, variation in degree of polymerization of the polymer material forming the organic semiconductor element and the organic semiconductor structure is an undesirable factor because it gives variations in property of the organic semiconductor element and the organic semiconductor structure. However, it is generally difficult to obtain a polymer material with a certain molecular weight, and is problematic from an industrial viewpoint.
The present invention has solved the problems described above. The object of the present invention is to provide an organic semiconductor material whose material designing is easy, and is capable to secure satisfying purity, so that it can be easily used industrially. And further, also to provide an organic semiconductor structure and an organic semiconductor device using the organic semiconductor material.
Up to now, it has not been known that a polymer material having a conjugated system molecule in its main chain, whose conjugation being cut, exhibits high charge carrier mobility. The reason such polymer material does not exhibit high charge carrier mobility is that, in a polymer material having a long conjugated system in its main chain, electron conduction occurs along its main chain, while in a polymer material merely having a short conjugated system in its main chain, charge is transferred by intermolecular hopping conduction because the conjugated systems among the molecules are overlapped. Conventionally, a material, which is capable to control the overlap of the short conjugated systems in main chains is, not known, and high charge carrier mobility could not be attained.